1 files changed, 23 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 7303ee2add2..6ba00bccdf8 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -29,12 +29,18 @@
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetMachine.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include <map>
 #include <iostream>
 using namespace llvm;
 
+static cl::opt<bool>
+SplitPHICritEdges("split-phi-constant-crit-edges", cl::Hidden,
+         cl::desc("Split critical edges for PHI values that are constants"));
+
+
 #ifndef _NDEBUG
 static cl::opt<bool>
 ViewDAGs("view-isel-dags", cl::Hidden,
@@ -959,6 +965,10 @@ unsigned SelectionDAGISel::MakeReg(MVT::ValueType VT) {
   return RegMap->createVirtualRegister(TLI.getRegClassFor(VT));
 }
 
+void SelectionDAGISel::getAnalysisUsage(AnalysisUsage &AU) const {
+  if (!SplitPHICritEdges)
+    AU.setPreservesAll();
+}
 
 
 bool SelectionDAGISel::runOnFunction(Function &Fn) {
@@ -966,6 +976,19 @@ bool SelectionDAGISel::runOnFunction(Function &Fn) {
   RegMap = MF.getSSARegMap();
   DEBUG(std::cerr << "\n\n\n=== " << Fn.getName() << "\n");
 
+  // First pass, split all critical edges for PHI nodes with incoming values
+  // that are constants, this way the load of the constant into a vreg will not
+  // be placed into MBBs that are used some other way.
+  if (SplitPHICritEdges)
+    for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
+      PHINode *PN;
+      for (BasicBlock::iterator BBI = BB->begin();
+           (PN = dyn_cast<PHINode>(BBI)); ++BBI)
+        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+          if (isa<Constant>(PN->getIncomingValue(i)))
+            SplitCriticalEdge(PN->getIncomingBlock(i), BB);
+    }
+
   FunctionLoweringInfo FuncInfo(TLI, Fn, MF);
 
   for (Function::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)